The Zoom button was introduced in Mac OS X, it didn't exist in any Apple operating system before that.

Kindly identify the box in the upper right corner of the frontmost window in this screenshot from 1988:

http://www.guidebookgallery.org/pics/tutorials/macplus1988/f...

Hint: the name is given at the bottom. Where did you get this crazy idea?

I stand corrected.

I also notice with some humor that back then, "Zoom" enlarged the window to full size :-)

I'm pretty sure that was just an easy description for the tutorial. It always sized to fit the window's contents, but that takes too many words to describe for the context.